Method for the protection of zirconium and zirconium-base alloys



United States Patent Int. Cl. c236 /06, 5/52 US. Cl. 204-37 5 Claims ABSTRACT OF THE DISCLOSURE The method of protecting zirconium and its alloys by the electrolytic deposition of chrome thereon, utilizing an electrolytic bath comprising an aqueous solution of from 400 to 550 g./l. CrO to 40 g./l. SrSO and 30 to 80 g./l. K SiF maintained at a temperature between 10 and 30 C. and at a current density of 5 to 40 A./dm.

My invention relates to the protection of zirconium and zirconium-base alloys [hereinafter generically called zirconium] against corrosion by external agents at high temperatures.

It is well-known that zirconium and zirconium-base alloys and particularly those alloys used in nuclear reac tors as cladding materials or in the fabrication of pressure or calandria tubes, have limited applications due to thecorroding action of the coolants, generally, presurized water, carbon dioxide, terphenyl or steam. Furthermore, the fabrication and transformation of the same alloys are diflicult and costly because of the rapid corrosion in air at temperatures above 800 to 900 C.

Due to external agents, a zirconium oxide film of 10W ductility is formed on the surface of zirconium. This film has a tendency to thicken and to eventually scale off. Simultaneously with the thickening and scaling off of the zirconium oxide film, oxygen penetrates into the subjacent metal and causes these areas to become brittle. This phenomena is even more pronounced as the temperature increases.

To protect zirconium against such corrosion, it has been proposed that it be separated from the external agents by coatings for which various coating processes have been developed. Different metals have been tried as a coating material including aluminum, copper, nickel, and iron. However, the deposition techniques have been inefficient particularly, where the metals are submitted to a high temperature during either utilization or transformation. The primary defects of the prior deposition techniques include unevenness or great thickness of the coating and its lack of adherence or adhesion, particularly, when hot. The maximum temperature at which the coating may be used is in adequate and limited because of the inner diffusion between the zirconium and the coating itself and/or to the formation of fusible eutectics. The specificity of the agent against which the coating was efficient has been too high. Finally, the prior processes are complex in nature, high in cost, and/or irregular in result. Not all of the prior techniques manifested all of these defects simultaneously, but at least one of them was present and generally several. 7

Of all the protective materials proposed for the protection of zirconium, chromium has shown the greatest advantages. Chromium oxidizes very slowly, is hard, and is resistant to abrasion. It has practically no diffusion into zirconium up to a temperature of approximately 800 C., and only slowly above this temperature. Furthermore, it reacts with zirconium to form a fusible alloy only at approximately 1300 C. Also of importance, is the fact that chromium has a coefficient of expansion very close to that of zirconium.

3,502,549 Patented Mar. 24, 1970 Prior to my invention, the depositing of chromium electrolytically on zirconium pieces had been tried but without success. The usual chromium plating baths contain ponderal ratios of CrO /H SO of approximately 100. The chromium deposits comprise spaced apart nodules which require the coating to be thick if it is to be continuous due to the progressive surface increase of the nodules and their bonding, Moreover, even when a contlnuous outer coating was obtained, thick coating had inherent defects of adhesion particularly at high temperatures. Accordingly, these electrolytically plated zirconium pieces have a tendency to blister and thus fail to protect the zirconium, particularly during deformation to which it is submitted.

I have invented a method for placing a tight and perfectly adhesive chromium coating upon zirconium and zirconium-base alloys. The electrolytic bath of my invention comprises an aqueous solution containing:

400 to 550 g./l. CrO preferably close to 500 g./l. 10 to 40 g./l. SrSO preferably between 20 and 25 g./l. 30 to g./l. K SiF preferably 60 to 65 g./l.

The temperature at the bath is maintained near room temperature, for example, between 10 and 30 C. The current density is maintained between 5 and 40 A./dm. preferably near 20 A./dm and the bath is stirred. The anode is a lead-base alloy anode, preferably a PbSb alloy containing 6-8 percent by weight Sb, and the interpolar distance is preferably between 3 and 200 millimeters.

Utilizing my method it is possible to obtain a continuous deposit with a minimum thickness of about 2 microns. For best results, a thickness of from 5 to 15 microns is placed on the zirconium, and it is possible to realize even thicker coatings with my process. Furthermore, my coatings have excellent adhesion. The coatings protect zirconium against most of the usual agents brought into contact with it at high temperature and especially against organic coolants. In particular, the coating protects against terphenyls up to around 400-450 C., against air, oxygen, carbon dioxide, up to 700-800 C. for long periods of time. The chromium coating applied on the zirconium with my method also protects for short periods of time against air, oxygen, carbon oxides and molten salt baths which have temperatures as high as 1000 to 1200 C. In fact, the coating may protect at these high temperatures up to several hours, Moreover, it is possible to quench the coating in oil or water without any fission or cracking.

It is also possible to obtain a coating which is both more adhesive and more ductile by subjecting the coating obtained to a vacuum-treatment between 700 and 850 C. This complementary treatment provides a greater margin of safety during hot deformations in air. During this thermal treatment a film of Cr-Zr intermetallic compound, usually below 1 micron but never above 5 microns is formed between the chromium and zirconium. This intermetallic compound insures a strong adhesion between the base metal and the coating. Interestingly, this vacuum-treatment considerably improved the ductility of the chromium deposit due to the elimination of the gaseous impurities contained therein.

The coatings of the invention protect zirconium and zirconium-base alloys when used at high temperatures, during hot deformations and during thermal treatments in air. In the latter case, the coating deposited according to my invention may be removed after thermal or quenching treatment.

The following nonlimitative examples illustrate the results obtained with a coating of this invention as against comon external agents.

3 EXAMPLE 1 Bulk cylindrical samples of zirconium-copper alloy with 1.6 percent copper, 50 mm. in length and mm. in diameter, were chromium plated by being used as a cathode in a stirred aqueous solution of:

CI'O3 K SiF 60 S1'S0 22 which constituted the electrolytic bath.

The electrolysis was carried out at C. with a 20 A./dm. current density. The anode was a cylindrical Pb-Sb tube with 6 percent Sb. The interpolar distance was between and mm. A chrominum film of about 10; was deposited within 20 minutes.

The coated samples were placed in an autoclave containing CO under atmospheric pressure at 700 C. After 3 days, their weight gain was ten times lower than that of identical noncoated cylinders placed under the same conditions.

EXAMPLE 2 A Zr-Nb sheet with 2.5 percent Nb, and having a dimension of 500 x x 1.5 mm. was used as a cathode in a stirred electrolytic bath of the same composition as that of Example 1.

The electrolysis was carried out at 15 C. with a 15 A./dm. current density. The anode was a Pb-Sn slab with 6 percent Sb. The interploar distance was 30 to 40 mm. The chromium film was deposited in 25 minutes and was approximately 12 thick. The coated sheets were placed in a furnace at 1000 C. in air for 15 minutes, then quenched in water at 25 C. Despite the thermal shock, the chromium coating neither blistered nor scaled, and the coating efficiently protected the alloy during the air treatment at 1000 C.

EXAMPLE 3 A Zr-Cu tube portion with 2.5 percent copper and being 500 mm. in length, about 13 mm. in outer diameter and about 11 mm. in inner diameter, was utilized. Under conditions similar to those in the previous examples, a chromium film of 10 was deposited by electrolysis on both the inside and outside of the tube.

After this electrolytic operation, the tube was maintained under vacuum for 30 hours at 800 C., then submitted to 40 successive thermal cycles of one hour each, comprising 20 minutes of heating. The heating temperature in air was 700 C. The cooling temperature was 40 C.

After examination, the chromium film was neither blistered nor scaled.

EXAMPLE 4 Strips of zirconium-base alloy containing:

Zr, remainder.

were coated electrolytically under conditions similar to those in the previous examples, but the deposit time was 40 minutes instead of 20 minutes, which permitted obtainment of a 20 micron chromium film.

The examination of a section of the strips showed the apparition between base alloy and chromium of an intermetallic film of about 0.5 1 in thickness.

The strips were placed into an atuoclave containing CO under atmospheric pressure at 700 C. In 6 days, their weight gain was ten times lower than that of noncoated samples of the same alloy placed under similar conditions.

The examination of the strips also showed that the chromium film was neither blistered nor scaled.

4 EXAMPLE 5 Tube portions of Zr-Nb alloy with 2.5 percent Nb having an mm. length, a 20 mm. outer diameter and an 0.5 mm. thickness were chromium plated electrolytically under the same conditions as those of Example 1, which permitted obtainment of a chromium film, about 10a thick both inside and outside.

After this operation, these tubes were heated in air at 1000 C., in a high frequency induction furnace. After 2 minutes at 1000 C., they were quenched in cold water. Under such conditions the chromium film was neither blistered nor scaled.

EXAMPLE 6 Tube portions of Zr-Nb alloy with 2.5 percent Nb having a 600 mm. length, a 92 mm. inner diameter and a 98 mm. outer diameter were plated. After the chrome plating, the tubes were immediately introduced into a metal furnace at 900 C., and maintained for 30 minutes at 900 C. and quenched in water at the end of the furnace. It was found that after quenching, the chrome deposited according to the invention was always present, and after a chemical removing of the chrome, the metal was free of oxide pits or defects.

EXAMPLE 7 A large piece of Zr-Nb (250 x 400 x 75 mm.) having a chromium film of 15 thickness deposited thereon according to the invention, was maintained for 1 hour, in air, at a temperature of 1200" C., prior to being rolled at a high temperature. It was found that the film had protected the piece against oxidation.

I claim:

1. A method for the protection of zirconium and zirconium-base alloys by electrolytic deposition of chromium, comprising:

(A) electrolyzing said zirconium in an aqueous electrolytic bath of from 400 to 550 g./l. CrO 10 to 40 g./l. SrSO, and 30 to 80 g./l. K SiF with a current density of from 5 to 40 A./dm. utilizing a lead-base alloy anode;

(B) stirring said bath; and

(C) maintaining the temperature of said bath between 10 and 30 C.

2. The method as set forth in claim 1 wherein said bath comprises 500 g./l. CrO 20 to 25 g./l. SrSO and 60 to 65 g./l. K SiF with a current density of 20 A./dm.

3. The method as set forth in claim 1 wherein said anode is a Pb-Sb alloy having 6 to 8 percent by weight Sb.

4. The method of claim 1 wherein the interpolar distance is from 3 to 200 mm.

5. The method of claim 1 including subjecting the coated metal to a vacuum-heat treatment from between 700 and 850 C. to form an intermetallic Cr-Zr compound of up to 5 thick between said coating and zirconium.

References Cited UNITED STATES PATENTS 1,608,694 11/1926 Cain 204-37 XR 1,745,912 2/ 1930 Richardson 20437 XR 1,838,273 12/1931 McBride 204--37 2,640,021 5/1953 Passal 204-51 3,041,257 6/1962 Cope et al. 2045l JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner U.S. Cl. X.R. 2045l 

